1. Field of Invention
This invention relates to holography and more particularly to a holographic recording medium comprised of a thermally deformable plastic.
2. Description of the Prior Art
In a typical hologram of an image, each portion of the image is recorded at a multiplicity of locations on a recording medium. Therefore, the hologram is inherently a highly redundant recording. Because of the inherent redundancy, when, for example, portions of the hologram are disfigured or discarded, an undiscarded, clean portion can provide a holographic reconstruction of the entire image. The inherent redundancy is a desirable feature of the hologram which is practially unknown in other types of recording.
The image may be holographically recorded on a medium, such as a tape, which typically includes a layer of a photoconductor intermediate to a layer of an electrically conductive material and a layer of thermoplastic. Accordingly, the thermoplastic has a surface abutting the photoconductor and a free surface.
The thermoplastic is comprised of a dielectric which becomes soft in response to an application of heat. Materials such as thermoplastic are known in the art as thermally deformable plastics.
To holographically record an image on the medium, a uniform voltage is maintained between the conductive layer and the free surface. In response to the photoconductor being provided with an interference pattern of light representative of the image, an electrical charge on the thermoplastic varies spatially in a pattern of a diffraction grating (comprising the hologram of the image).
The pattern of the charge establishes a corresponding pattern of electrostatic force between opposed surfaces of the thermoplastic. In response to an application of heat, the free surface is softened whereby the diffraction grating is formed therein by the electrostatic force.
When the diffraction grating is used to provide a holographic reconstruction of the image, portions of the image are usually inaccurately reconstructed. The inaccurate reconstruction is caused by a correspondence of portions of the image to spatial frequencies which are not within a spatial frequency pass band of the thermoplastic whereby the portions are not efficiently recorded. Typically, the pass band is approximately equal to 400 cycles per millimeter. Additionally, the pass band has a center frequency known in the art as a frost frequency. It is well-known that the frost frequency is inversely proportional to the thickness of the thermoplastic.
Usually, the 400 cycles per millimeter pass band is on the order of a range of spatial frequencies comprising the hologram of the image (referred to as the image range hereinafter). Therefore, it is desirable to have the frost frequency substantially equal to the center frequency of the image range. Since the frost frequency is inversely proportional to the thickness of the thermoplastic, the thickness is of critical importance when the thermoplastic is fabricated; a slight error in the fabrication which causes an error in the thickness results in the front frequency deviating from the center of the image range thereby causing an inefficient recording. The critical importance of the thickness causes the recording medium to be expensive and unreliable. Heretofore, the inherent redundancy of a hologram has not been used to broaden the spatial frequency pass band of a layer of thermoplastic.